How To Watch Sunday's Rare Supermoon Eclipse, The Last For Almost 20 Years

This coming Sunday night (September 27), observers around the world are set to be in for a treat as two celestial events combine. In a rare occurrence, the Moon will reach its closest point to Earth (known as a supermoon) at the same time as it undergoes an eclipse, the first such alignment since 1982 – and the last until 2033.

During the event, when the Sun will be directly behind us in respect to the Moon, the lunar surface will appear a deep red color. Lunar eclipses are fairly common, with 16 occurring this century already, but only five supermoon eclipses have taken place since 1900.

It will be best seen at every longitude from the U.S. to Europe, encompassing Central and South America, the U.K. and Africa. Observers in western Asia and Alaska will have a lesser view. Check out this map at timeanddate.com to see the best locations.

The entire event will last about five hours, but there are only about 70 minutes when the Moon will seem red. At 8:11 p.m. EDT on Sunday, the Moon will begin to enter Earth’s shadow, giving it a slight yellowish hue. From 10:11 p.m. to 11:23 p.m. EDT, the eclipse will reach totality, making the Moon appear a deep red, and at 1:23 a.m. EDT on Monday, it will exit Earth’s shadow.

For observers in the U.K. and Europe, you might want to take the morning off work to catch a glimpse of this rare event. The Moon will start entering Earth’s shadow at 1.11 a.m. BST, reaching the center at 3.11 a.m. BST and exiting at 6.23 a.m. BST.

The reason the Moon appears red, sometimes labeled a “blood moon,” is due to the refraction, or bending, of sunlight in Earth’s atmosphere. Think about a sunrise or sunset – the more atmosphere light has to pass through, the more it refracts towards the red end of the electromagnetic spectrum. During a lunar eclipse, light from the Sun refracts through the atmosphere and hits the lunar surface, making it appear red.

As mentioned, this eclipse is particularly special because it coincides with a “supermoon.” The Moon’s orbit around Earth is not perfectly circular; it ranges from 363,000 kilometers (226,000 miles) to 406,000 kilometers (252,000 miles). On Sunday night, it will be near its closest point, appearing 14% larger than an average Moon.

To the naked eye, this size difference isn’t very noticeable. If you see the Moon near the horizon, though, you may notice that it appears huge. This isn’t due to its distance, but rather an effect known as the Moon illusion, where your brain perceives the Moon to be closer at the horizon than up above, thus overcompensating and tricking you into thinking it is bigger.

This NASA video explains how a supermoon lunar eclipse works.

This event is also the last of a sequence of four total lunar eclipses, which is known as a lunar tetrad. The first was on April 14, 2014, the second October 7, 2014, and the third April 4, 2015. These are considerably rarer than eclipses by themselves – only 50 lunar tetrads have occurred in the past two millennia.

If you aren’t in a location to see the eclipse this Sunday, then we’d recommend tuning in to NASA TV (embedded below), which will be live streaming the event.